1. Field of the Invention
The present invention relates to a method for using oxycombustion to heat a substance present in a heating zone, such as the distribution canals or forehearths particularly those used for conditioning glass.
The burners are commonly used as a heating system for heating a substance present in a heating zone.
In this context, the term heating covers not only the addition of energy to a substance in order to increase its temperature, but also methods of supplying energy with a view to maintaining or controlling the temperature of this substance, for example in order to obtain or maintain a more homogeneous temperature through the substance.
2. Related Art
A burner is a device used to combine a fuel and an oxidant with a view to burning them. The fuel and the oxidant are usually carried to the heating zone through the burner from outside the enclosure defining the heating zone.
Air is the oxidant traditionally used. Burning using air as an oxidant is known as air combustion.
In order to increase the energy efficiency of the burners and reduce the amount of pollutants, notably of NOx, produced, the air as an oxidant may advantageously be replaced with oxygen-enriched air or with more or less pure oxygen. Combustion is then known as oxycombustion and the corresponding burners are oxyburners.
This change in oxidant has the effect of altering the characteristics of the combustion and especially those of the flame.
When changing from air combustion to oxycombustion the flame temperature becomes higher. The radiative heat transfer becomes more intense and occurs in part in a different range of wavelengths. All of these factors play a part in improving the heat transfer between the flame and the substance that is to be heated.
Depending on the target application, the burners are chosen according to their power and/or according to the shape of their flame.
For example, in the case of a melting furnace, the burner or burners provide the solid charge with the heating and melting energy needed for melting. For such an application, use is therefore generally made of high-power burners the flames of which are able to cover a significant proportion, if not all, of the free surface of the substance that is to be melted in the relevant zone of the furnace.
In the case of the distribution canals which transport the molten substance from the melting furnace to the installations downstream, such as shaping machines in particular, the burner or burners supply the energy necessary such that on entering the downstream (shaping) installation, the molten substance has homogeneous properties such as temperature, viscosity, etc. suited to its treatment in the downstream installation. The distribution canal may in particular be equipped with burners aimed at compensating for heat losses through the walls of the canals so as to reduce the heterogeneity, notably in terms of the temperature of the molten substance in the transverse section of the distribution canals. The burners of the distribution canals are then, as a general rule, low-power burners the flame of which is limited to a zone near the walls of the canal.
What is more, in the case of a flame in a distribution canal which extends beyond the zone near the walls of the canal and which therefore also heats the molten material in the central zone of the canal there is a substantial risk of reboiling the molten material in this zone and therefore of having an end product that exhibits inhomogeneities and corresponding defects.
The power of a burner and the shape of the flame obtained are essentially dependent on the concept or design of the burner and on the nature and flow rates of fuel and oxidant.
The injector (or tip) of an oxyburner is generally made of metal (steel or refractory steel for example) and is often positioned inside a refractory port block.
For industrial applications, the durability and reliability and the performance in terms of energy consumption are particularly important properties for a burner.
Furthermore, it is desirable to be able to use one and the same burner or one and the same type of burner at different powers according to the technical characteristics of the method in which the burner or burners are used and/or to be able to take account of the ageing of the chamber defining the heating zone.
In practice, it is found that the high temperatures encountered during the oxycombustion heating method limit the durability (life) of the burner when the burner is not used correctly.
For example, an excessive increase in the temperature of the burner tip may lead to rapid degradation thereof requiring the burner to be taken out of service. To limit this risk, the tip of the burner is generally set back in the port block.
However, this configuration may also have the effect of very significantly increasing the temperature of the port block and of forming hot spots in the block and/or of causing the block to melt in places such that the port block and the burner both have to be taken out of service.
Another problem encountered with this type of burner is a lack of reliability or of consistent performance in terms of energy consumption which is due to the formation of deposits.
What happens is that the formation of soot, with carbon deposited on the fuel injector, is often accompanied by a change in the position of the flame given the change to the centering and/or the geometry of the flame; and therefore by a modification in the effectiveness of the heating method.
Moreover, such a modification in the position of the flame caused by soot forming on the tip of the burner may also lead to an overheated or melted zone on the block and therefore limit the durability of the block and sometimes also of the burner.
Finally, volatile substances from the heating zone may be deposited in the block or on the tip of the burner. A substantial build-up of volatile substances in the block/on the tip may also deflect the flame which, as explained previously in the case of carbon deposits, reduces the reliability of the burner and possibly also even its durability.
It is an object of the present invention to address the aforementioned disadvantages of burners by means of a heating method and system that are optimized and can be used for heating a molten substance in distribution canals.
DE-A-102005005735 describes a method for heating an industrial furnace by means of a burner of tube-in-tube type. The burner has a central fuel injector surrounded by an annular gaseous-oxidant injector, and a cylindrical chamber known as the mixing and combustion chamber between the injectors and the outlet opening of the burner. The combustion which begins in the cylindrical chamber (precombustion) is delayed because of similar impulse densities of the fuel and of the oxidant. The limited internal volume of the burner chamber, its cylindrical shape and its diameter which is close to the diameter of the outer pipe of the burner limits the degree of precombustion in the chamber and prevents turbulence and recirculation of the gases in the chamber as occurs with conical burner chambers. A high total impulse of the oxidant and of the fuel leaving the burner and the selection of the power density at the outlet from the burner mean that a substantial amount of flue gases from the furnace is sucked into the reaction zone of the flame, lowering the flame temperature and reducing the formation of NOx and of CO. The result is uniform heating of the charge in the furnace. The method according to DE-A-102005005735 is therefore particularly well suited to melting furnaces such as glass melting furnaces.
However, this method is not very well suited to applications in distribution canals where, as mentioned previously, heating located only near the walls is desired.